1. Field of the Invention
The present invention relates to multiconstituent fibers and their preparation, and to nonwoven structures prepared from such fibers.
2. Description of Background and Other Information
Multiconstituent fibers, and means for their preparation, are known in the art. References in this area include U.S. Pat. No. 3,616,149 (WINCKLHOFER), U.S. Pat. No. 4,634,739 (VASSILATOS '739,) U.S. Pat. No. 4,632,861 (VASSILATOS '861, a division of VASSILATOS '739), U.S. Pat. No. 4,839,228 (JEZIC et al. '228), U.S. Pat. No. 5,133,917 (JEZIC et al. '917, a continuation of JEZIC et al. '228), and U.S. Pat. No. 5,108,827 (GESSNER).
Various known methods, of preparing multiconstituent fibers, include procedures which involve dry blending, then extruding the polymers, or subjecting the dry blended polymers to melting, and possibly additional blending, before extrusion. In these methods, the polymers are invariably blended before melting is effected; accordingly, separate melting of the individual polymers does not occur.
Because the prior art processes do not employ separate melting of the polymers, prior to their blending, intimate mixing of the polymers is invariably effected, before the extrusion step which provides the fibers. Consequently, the domain size of the dispersed polymers is limited in one or more dimensions; for instance, the domains are narrow or fine, relative to the width of the fiber--e.g., they do not, individually, occupy much of the fiber cross-sectional area, or they have a small equivalent diameter, in comparison with that of the fiber--and/or they are short--i.e., they do not extend for a long distance, along the axis of the fiber.
For instance, among the results obtained, in the prior art processes, are continuous/discontinuous phase dispersions with the discontinuous phase provided in domains which typically have a width of less than one micron, at their widest point in cross-section, along the diameter of the fiber, or which have a cross-section no larger than 0.1 percent of the fiber's cross-sectional area. Further, where the miscibility or melt viscosity of the discontinuous phase component is widely different than that of the continuous phase component, the former can end up present in the form of discrete short fibrils, typically of less than 10 microns in length.
The fibers obtained from these prior art processes lack availability of the lower melting point polymer, on the fiber surface. In consequence, they fail to provide good thermal bondability between fibers.
As indicated, the prior art does not disclose or suggest, in the preparation of multiconstituent fibers, prior and separate melting, of the individual polymers, before their blending. The prior art further does not disclose or suggest, along with such prior, individual melting, moderating the degree of subsequent blending, and, if necessary, the initial relative amounts of the polymers, so that the ultimately resulting multiconstituent fiber is characterized by larger polymer domains than are provided by the prior art processes.
In this regard, it has been discovered that prior, separate melting, of the individual polymers, inhibits, or retards, the mixing of the polymers in the subsequent blending. Appropriate limitation of the amount of mixing, in such subsequent blending, and corresponding control of the relative amounts of the polymers employed, prevents the polymers from being broken up to the degree which is provided in the prior art, and results in the macrodomains, of the multiconstituent fibers of the invention.
The multiconstituent fibers of the invention provide novel and unexpected advantages, over those in the prior art. As an example, the presence of the polymer macrodomains effects superior bonding of the fibers, in the preparation of nonwoven structures or fabrics, particularly where low pressure thermal techniques are employed.
Such superior bonding especially occurs where the fibers of the invention comprise immiscible, or at least substantially immiscible, thermoplastic polymers of different melting points--whereby the application of heat melts the lower melting point components of the fibers, and the intermelding of such components, among the fibers, effects their bonding--and, more especially, where the at least two polymers are present in unequal amounts by weight, and the polymer present in the lesser amount is that having the lower melting point. As a particularly preferred embodiment, the superior bonding is realized in linear polyethylene/linear polypropylene multiconstituent, especially biconstituent, fibers of the invention, where the polyethylene is the lower melting point and lesser amount component.
As another advantage, the fibers of the invention can be thermally bonded without the use of any applied pressure, thereby resulting in lofty nonwoven structures, suitable for filtration, and other applications. Such superior low pressure thermal bondability particularly results where the fibers of the invention feature at least two polymers of different melting points, with the lower melting of these polymers provided as macrodomains; in this instance, the indicated favorable bondability is effected by the availability of the lower melting polymer component--due to its macrodomain dimensions.